In the production of a thin cast article having a thin portion of 5 mm thick or less, the fluidity of a melt introduced into a mold cavity is rapidly decreased because a part of the melt is rapidly cooled and easily solidified upon coming into contact with the internal wall of the mold cavity. This results in defects such as insufficient filling of the mold cavity, etc. In the production of a cast article of complicated shape, air and gases generated from the mold material are likely to be introduced into the resulting cast articles as defects such as blow holes. Thus, a defectless cast article which is thin and has complicated shape is difficult to be produced.
As a method of producing a thin cast article having complicated shape, a lost wax process has been known. In this method, a ceramic mold is heated to 700.degree.-900.degree. C. prior to the filling of the cavity with a melt to retard the cooling of the melt introduced into the cavity, thereby keeping the melt highly flowable. However, since a ceramic mold is expensive, the production cost of a thin cast article having a complicated shape would be extremely high.
As an alternative method, JP-A-60-56439 discloses a gypsum mold provided with a cavity, runner, etc., in which a refractory filter having a gas permeability higher than that of the gypsum is disposed in an area ranging from the neighborhood of a last-filled portion of the cavity to the outside surface of the gypsum mold, thereby enhancing the evacuation capability to increase the fluidity of the melt and prevent the defect due to gas. The gypsum mold is produced by hydration-setting a gypsum slurry and drying the hardened gypsum. This method utilizing the gypsum mold, as is the case of the lost wax process mentioned above, has been known as one of the precision casting methods for producing a cast article of a high dimensional accuracy, and has been used for producing dies, parts for machines, artistic handicrafts, etc.
However, since the production of a gypsum mold includes the steps of kneading, pouring, hydralion setting, pattern draw, drying, etc. takes a long period of time, over 48 hours, the productivity of this method is poor. Further, since the gas permeability of the gypsum mold is extremely low, it leads to difficulties in determining the casting design for evacuation and pressurization at filling of a cavity with a melt. In addition, since the cooling rate of a gypsum mold is low, the melt in the mold solidifies extremely slowly. Therefore, in the case of casting a thin article of complicated shape, a shrinking defect is likely to occur, resulting in a low yield of the desired cast article.
Recently, a vacuum casting method as disclosed in JP-B-60-35227, etc. has come to be used. In this method, a melt is introduced into a mold cavity by evacuating a mold. However, in this method, air is likely to be incorporated into the melt through a mold portion which is not immersed in the melt, failing to obtain a sufficient vacuum. Further, although it is applicable to casting of articles of low height and simple shape, it is difficult to be applied to casting of high and thick articles of complicated shape.
JP-A-64-53759 discloses an apparatus in which a mold provided with a runner passing through the mold is disposed in a vacuum vessel, the upper end of the runner being closed with a stopper which does not pass a melt through it. A mold cavity, runner, etc. is filled with a melt by adjusting the pressure applied on the upper end of the runner passing through the mold lower than the pressure of the interior of the vacuum vessel surrounding the mold. However, in this art, since the vessel is evacuated through a hole positioned above the sprue, a sufficient vacuum can not be achieved at last-filling portions of the mold cavity, riser, run-off, etc.
JP-A-2-303649 discloses a vacuum casting method in which a mold having rammed particulate matter around it is maintained in a vacuum vessel by virtue of vacuum, and the mold is immersed into a melt thereby introducing a melt into the mold. In this method, however, since a mold is immersed in a melt together with the rammed particulate matter around it, the melt is disturbed before and after the immersion of the mold to cause an incorporation of air into the melt. Further, since a mold and rammed particulate matter around it project out from the vacuum vessel, air is likely to be incorporated into the melt from the bottom portion of the mold.
As mentioned above, the filling of a mold cavity with a melt is insufficient in the prior art method. In particular, the casting of a thin cast article, 5 mm thick or less, particularly 3 mm thick or less, having a complicated shape is difficult in the prior art method.
Accordingly, an object of the present invention is to provide a vacuum casting apparatus suitable for producing a cast article, particularly a thin cast article having a complicated shape, without suffering from casting defects such as insufficient filling, blow holes, etc. in a good productivity.
Another object of the present invention is to provide a vacuum casting method which shows the effect mentioned above.